It is known that the oxygen content of liquids and gases can be determined by polarographic measurements in which oxygen is reduced at a polarographically active cathode. Measurement of the cathode current provides means for determining the partial pressure of oxygen(pO.sub.2) in the fluid. Ordinarily the measurement requires a cathode area small enough that the current is unaffected by the linear flow rate of the liquid. With large cathodes spurious results occur at low flow velocity because the liquid adjacent to the cathode surface becomes so depleted of oxygen that the reduction current does not depend entirely on the oxygen concentration, but also on the rate at which the liquid moves to and from the surface. As the flow velocity increases the current also increases to a plateau level, which is a function of the pO.sub.2.